Public address system



June 12,:1951 J, F, STERNER 2,556,889

I/ l PUBLIC ADDRESS SYSTEM Filed may 29, 1948 Patented June 12, 1951 PUBLIC ADDRESS SYSTEM John F. Sterner, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application May 29, 1948, Serial No. 30,139

9 Claims.

The present invention relates to public address systems and the like, wherein one or more loudspeaker devices are provided with highly amplified audio frequency currents from a microphone positioned in substantially close proximityv to the loudspeaker or loudspeakers for facilitating'sound distribution to a, large audience and to magnify the voice of the person addressing the audience.

rBecause of the close proximity of the microphone to the loudspeaker arrangement of a public address system, and because lthe gain of the amplifying means may at times be adjusted to a high level for reaching a maximum coverage of an audience, acoustical feedback to the microphone from the loudspeaker or loudspeakers may occur. This results in audio frequency oscillations which completely interfere with the normal transmission of desired speech or sounds and its prevention requires constant supervision or monitoring to maintain the gain level below the feedback point.

As is well known, acoustical feedback also may occur when a speaker approaches too closely to a microphone, thereby raising the sound output level at the loudspeakers and correspondingly raising the feedbacksound level at the microphone, resulting in sudden acoustical feedback and interrupting the normal use of the public address system until the condition can Abe corrected..

It is, therefore, a primary object of this invention, to provide an improved public address system of the type referred to, wherein acoustical feedback in the useful audio frequency range for the transmission of sounds may be prevented while maintaining a desired adjustable and high gain level in the system.

The usual public address system to which the invention relates may include a microphone connected to a suitable electronic tube amplifier having variable or adjustable gain control means therein, a power amplifier coupled to the gain control ampliier, and one or more loudspeaker devices distributed throughout the area to be covered by the system from the single sound source represented by the microphone. Other microphones may be connected in parallel with the i 2 tored by an operator who is responsible for maintaining the gain of the electronic tube amplifier system at such a level that maximum coverage is obtained, that is, maximum sound output from the loudspeaker system is maintained while4 preventing the level from reaching a point at which one microphone, directly or through suitable amplifying means, but the net result is the same as if a single source of sound were provided, and in any case the microphone or microphones are subject to acoustical feedback from the loudspeaker devices because inherently the person addressing the audience and the loudspeakers must be located adjacent to the audience and, therefore, at substantially the same location acoustically. This fact accounts for the difficulty which is experienced with all types of sound ldistributing systems ofthe public address type to prevent acoustical feedback in use.

Such .systems are, therefore; generally moniacoustic feedback may occur. Generally this point must be estimated at all times. Due to the constantly changing position of a person addressing an audience, with respect to the microphone, monitoring of the gain, therefore, is dificult. At times thelevel must be reduced as the speaker approaches the microphone and at other times proper reproduction may require the full power of the amplifier, as when the speaker withdraws Suiiiciently from the microphone While talking. Therefore, even though monitored, the amplifying system may often overload or overshoot, and acoustic feedback will result in spite of every precaution, thereby interrupting the message delivered by the speaker and seriouslyinterfering with the enjoyment of the talk or address.

It is, therefore, a still further object of the invention, to provide an improved electronic tube amplifying system for public address systems and the like, which operates automatically to suppress acoustic feedback and to maintain the gain of the amplifying system below a predetermined level regardless of the adjustment of the gain by manual means under control of an operator and regardless of the position of the speaker with respect to the microphone.

It is also a still further object of the invention, to provide an improved public address system provided with gain control means for monitoring purposes which includes warning means for an operator to indicate a condition of operation wherein the gain is at a point where the amplifying system may break into acoustic feedback.

A further object of the invention is to provide an improved ampliiier for public addressk sys-` tems and the like, having automatic gain controL means for preventing audio frequency or acoustical feedback to occur in operation, togetherl with means for adjusting the gain level as desired independently of the acoustical feedback, whereby,

inexperienced and unskilled gain control attend-` ants may be employed for the operation of the system.

In accordance with the invention, a dual channel electronic tube amplifier is provided in connection with a common output amplifying means therefor and a common or master gain control means.

Separate input or pickup microphones are provided for each channel, one of which is responsive to the useful audio` frequency range for the transmission of sounds through the system, for example between and 10,000 cycles and hereinafter referred to as the sonic range,A

and the other channel being responsive to supersonic sounds in an adjustable narrow range widely separated in frequency from the sonic range, for example in a narrow supersonic frequency range including 11,000 cycles. The acoustical feedback in the supersonic range is adjusted to exceed that in the sonic range and a gain controlling potential for the sonic amplifying channel is derived from the supersonic amplifying channel and ad- .iusted to maintain the gain in the sonic channel always below a maximum gain level without acoustic feedback. With the acoustical feedback confined to the supersonic range, feedback control may be provided automatically without interfering with the transmission of sonic frequency signals or normal sound through the system.

The invention will, however, be further understood from the following description when considered in connection with the accompanying drawing and its scope is pointed out in the appended claims.

In the drawing:

Figure 1 is a schematic circuit diagram of a public address system embodying the invention, and

Figure 2 is a graph showing certain operating characteristics of the system of Figure l.

Referring to Figure l, a public address system is shown wherein a plurality of sound producing devices or loudspeakers 5 and E are positioned to provide proper coverage of an audience and are supplied with audio frequency signals through a power amplifier 1 from one or more microphones, indicated by the sound pickup microphone 3, and with a signal amplifying channel 9 interconnecting the power amplifier and the microphone 8.

The signal amplifying channel 9 comprising a suitable preamplifier I connected directly with the microphone 3 through the usual cable connection II permitting positioning of the microphone at any desired point, and the amplifier I8 is coupled as indicated, through a coupling capacitor I2, with the input or grid circuit I3 of a variable gain electronic amplifier stage including an electronic tube I4. The output circuit I5 of the variable gain amplifier stage is coupled through a low pass filter network I3 and a coupling capacitor I1, with a variable gain control or potentiometer device I8 having a variable gain control output circuit connection I9 with the grid or input circuit of a second variable gain amplifier stage 2e. This amplifier stage in turn, is coupled through an output circuit 2i with the power amplifier 1.

The low pass filter I8 may comprise a series impedance device, such as a resistor 25, and a shunt capacitor 23 across the signal channel. In the present example, this is designed to pass signals generally below the upper' limit of the desired sonic range, which may be considered by way of example to be of the order of 10,000 cycles. This amplifying channel is thereby made responsive in the sonic or useful audio frequency range for the transmission of speech and other audio frequency sounds to be conveyed from the microphone 8 to the loudspeaker system 5 5.

The gain of the amplifier stage 23 is variable by means of the potentiometer device I8 which may be controlled manually by an operator of the system, to maintain the sound level at the loudspeakers suiciently high to cover the audience and to maintain the gain level at such height that the variable sound pickup from the microphone `8 is compensated to maintain substantially phones 8 and 3| are delivered to the terminal 43 and the gain control device I8 as a master gain a constant level at the loudspeaker by means of the monitoring.

Assuming the gain of the amplifier stage I4 to be fixed, it will be seen that with such a public address system, acoustic'feedback may at times occur between the loudspeakers 5 and E and the microphone 8, since they are inherently positioned in close proximity one with respect to the other for public address purposes. Such feedback may normally be prevented by careful monitoring of the program at the gain control device I8, whereby sound output of the loudspeakers is maintained below a level causing acoustical feedback. However, such a system is subject to all of the disadvantages hereinbefore referred to, such as overshooting and over-monitoring in either direction in response to variations inthe sound level at the microphone 8 by reason of the movement of the speaker with respect thereto during an address. Y

Further in accordance with the invention, therefore, a second signal amplifying channel 30 is provided in parallel with the channel 9 between the gain control device I8 and a second microphone 3I, which is positioned adjacent to the microphone 8 in operation, or in such position that it is subject to acoustical feedback from the loudspeakers 5 and 6.

The second amplifying channel is provided with a preamplifier 32 connected with the microphone 3l through an input cable indicated by the lead's` 33, and the preamplifier, in turn, is coupled' through an output circuit 34 and a coupling capacitor 35 with a variable gain control device 36 for an electronic tube amplifier stage represented by the amplifier tube 31. The grid circuit 38 of the tube 31 is connected to the output of the variable gain control device 33 which, like the gain control device I8, may be a potentiometer device of the resistor type, as indicated.

The output circuit of the amplifier stage 31 is indicated at 40, and is connected, through a coupling capacitor 4I and a high pass filter network 42, with an input connection 43 on the variable gain control device I8 in common with the input connection from the main or sonic amplifying channel 9. With this arrangement, signals from both channels 9 and 30 and from the microcontrol device for the amplifying system. Therefore, the sound level at the loudspeakers for both channels is simultaneously adjustable by the master gain control device I8.

The high pass filter network 42 is designed to transmit through the second amplifying channel 30 only such signals as lie outside the sonic range hereinbefore referred to and preferably in a narrow band relatively widely separated in frequency therefrom, such as a narrow band between 16,000 and 18,000 cycles centering about a 17,000 cycle normal response. In the present example, the high pass filter comprises a series resistor 45 and a shunt capacitor 46 connected across said resistor, the capacitor having a value suflicient to pass high frequency signals above the sonic or audible range across the impedance of the resistor 4'5.

To further accentuate the response of the am'- plifying channel 30 in the narrow supersonic band above referred to, the channel may further be peaked or made resonant in that band, by means of a supersonic resonant circuit network as indicated at 41. This comprises a tuned choke i coil 48 connected in shunt across the output circuit 4D through a bypass capacitor 49 to ground, While anode potentials are applied to the ampliiier stage through a lead 59 in which the choke coil 48 is connected as shown. The choke coil may be tuned by a variable shunt capacitor 5I or by any other suitable means, thereby completing the resonant circuit which is arranged to have a relatively high impedance in the supersonic frequency band selected. Likewise, the microphone 3| and the preamplifier 32 may be made highly responsive to signals in the supersonic range and to suppress signals in the sonic range. From the foregoing description, it will be seen that the amplifier stage 31 is made variable in gain by connection with the variable gain control device 36 at the input end of the supersonic amplifying channel 30. It will further be noted that the variable gain stage I4 for the sonic channel is placed under control of the signal amplitude in the supersonic channel, through a gain control circuit from the grid or input circuit I3, connected to a variable tap 56 on an output impedance or resistor 51 for a signal rectiier comprising an anode electrode 58 and a cathode electrode 59 provided in the electronic tube 31 or in any other suitable manner for receiving and .rectifying signals derived from the supersonic dicated at 63, A bypass capacitor 64 having lowV reactance to 17,000 cycles is connected across the impedance provided by the resistor 51 and the indicator 62 in series.

With this arrangement, a D.-C. biasing potential is developed across the resistor 51 in response and proportional in amplitude to supersonic signals received at the microphone 3|, and this biasing potential is applied through thecircuit 55 to the grid circuit I3 of the variable gain controlling stage I4 in the sonic amplifier channel 9 in a direction toreduce the gain therein in proportion to an increase in the amplitude of the supersonic signals. The amount of biasing potential applied to the stage is controlled by adjustment of the contact 56 on the resistor 51, and this circuit is further filtered to eliminate coupling between the channels at the supersonic and sonic frequencies, by a shunt bypass capacitor indicated at 65 and a series filter impedance or resistor 66.

Referring now to Figure 2 along with Figure l, the operation is as follows: The gain of the sonic amplifier channel 9 is such that with the master gain control device I 8 adjusted for maximum output, acoustical or audible feedback may be obtained in the normal use of the system for public address purposes, that is, with the loudspeakers 5 and 6 at the same location generally as the microphone 8 but spaced therefrom in the usual manner for such installations.

The gain at the master gainV control element I 8 is then reduced to a level below which the system will operate Without acoustic feedback. This level is indicated in the graph of Figure 2 by the horizontal line 10 with reference to the relative amplitude and frequency coordinates shown. The normal frequency range` of response and the normal full gain level of the main amplifying or sonic The rectifier,

channel 9 is represented by the rectangle 1 I, indicating a frequency range of from approximately cycles to 10,000 cycles as the useful audio frequency response range for the sound or public address system through the channel 9.

'I'he relative response range for the supersonic amplifying or control channel 38 is represented by the lines 12 and 13 in the graph of Figure 2, indicating a general response centered about 17,000 cycles. The rectangle formed with the base line and the line 1I) between lines 12 and 13 forms a rectangle representing the general maximum response for the supersonic channel 38 without acoustic feedback from the loudspeakers 5 and 6 to microphone 3 I.

If now the master gain control I8 is adjusted for normal full gain level, as indicated by the horizontal line 1I, slightly below the maximum gain level 10, the supersonic gain control device 36 may then be adjusted to bring the gain of the supersonic amplifying channel 30 up to the line 10 or maximum gain level without acoustic feedback. If now with this arrangement, the master gain control is advanced to increase the gain so that the gain in the supersonic or control channel may be represented by the level at 14, for example, the gain of the main channel 9 will be then raised correspondingly and just to the pointof breaking through the maximum gain level which would normally provide acoustic feedback. At this level, the amplifying system will operate satisfactorily without audible feedback, although the supersonic amplifier is then operating with a degree of supersonic feedback in the supersonic range but so far above the sonic range that it' is not audible to the normal ear. A certain portion of the supersonic energy is now derived from the circuit 48 through rectifier 58-58 and appears as a control voltage applied through the circuit 55 from the adjustable con'- tact 56 to the grid circuit i3 of the variable gain amplifier stage 4-, and the contact 56 is so'` adjusted that the resulting increase in negative bias on the amplifier I4 and a, corresponding reduction in gain isat a threshold value.

Further increase in gain at the master gain control I8, as by shifting the gain so that supersonic feedback Occurs at levels indicated at 15 and 15 for example, results in a corresponding increase in amplitude or volume of the supersonic feedback energy through the channel 30, and a correspondingly increased negative bias potential applied tothe variable gain stage I4 in the sonic channel. This control potential is of such value, by adjustment of the contact 56, that the gain in the sonic channel 9 is maintained just below the maximum gain level without acoustic feedback, that is, below the level represented by the line 'I0 in Figure 2. With.l

this arrangement, therefore, inadvertent or careless variation of the gain at the master control, beyond the maximum limit permissible Without acoustic feedback, results only in an intensified supersonic feedback which is inaudible and which holds, through the rectifying circuit and gain control stage i4, a normal gain level in the main or sonic channel below the maximum gain level, indicated by the line 10, so that in no case does any audible acoustic feedback occur in the system.v

The operator in ccntrol of the master gain control device IS for monitoring gain may be appraised of the approach into the feedback rangeV by the presence of supersonic feedback through the channel 3) as indicated by the rectification,

of signal energy iiowing through circuit 48. This assasse provides a D.C. current which flows in the rectifier output circuit including the resistor and the indicator device B2.

The indicator device 5.2 may be a sensitive microammeter or the like, the indications of which provide the operator with the desired warning and at the'same time indicate the amplitude of the feedback which will be proportional to the extent to which the system is being overdriven at the time and, accordingly, corrections maybe made to bring the indicator substantially to. zero, whereby the supersonic and sonic feed back is maintained substantially zerov and the system as a whole is operated below or not higher than the maximum level indicated by the line 'i0 in Figure .2. Any other suitable indicator, however, may be provided for this purpose which is resgonsive to current fiow in the rectifier circuit.

From the foregoing description it will be seen that a public address system in accordance with the invention inciudcs two amplifying channels which are coupled tca common variable gain electronic tube amplifier stage providing a com mon output control unit for both channels and a common variable gain control means preceding the usual power amplifier system for the loudn speakers used.

Furthermore, it will be seen that each of the separate amplifyingV channels is` responsive to signals in a different frequency range and each includes a variable gain amplifier stage, the variable gain stage in the sonic or main channel being dependent upon the signal amplitude in the supersonic or control channel in such a manner that the signal gain through the sonic channel is always maintained below a level at which acoustic feedback may occur between the loudspeaker output and the main input for that channel, and that the variable gain control is provided thrcugh the use of supersonic energy, that is, energy above normal audibility and the useful audio frequency range for which the main channel is provided.v

While the main response of the power amplifier and loudspeaker system is greatest in the audible frequency range, the overall response of the system through the master gain control stage 2i), the power amplifier 'i and the sound producers 5 and 6 includes an appreciable response at supersonic frequencies including the control band for the channel 30. The frequency response of the power amplifier and the speaker system there for or in any case the amplifier channel common to both of the parallel channels 9 and 3o will include the full audio frequency band for the channel e and an additional response which will include the frequencies conveyed by the control channel 3i). It is, therefore, contemplated that in such a system as shown the power amplifier 1 and a loudspeaker system will generally be responsive to the full frequency range from 100 cycles, for example, through 20,000 cycles, which amplifier and speakers are generally available for this purpose.

There is, therefore, no audible acoustical feedback at any time regardless of the position of the speaker with respect to the microphone or of the extent to which the master gain control is advanced in monitoring the program. In addition, the operator of the system is warned of the approach to the acoustical feedback range and may normally take action to prevent the further advance of the gain since, with the system adjusted as described, the gain through the sonic amplifier maximum gain level as shown in Figure 2. The system, therefore, operates to provide amplitude suppression in the main channel from suchV levels as indicated at 75 and i5, for example, in Figure 2 to the level indicated by the line 7G, which is the maximum gain level permissible for satisfactory operation of the sonic or main channel.

While a frequency of the order of 17,090 cycles has been chosen for the center of the supersonic control frequency band, it is obvious that any other suitable frequency band above audibility and substantially widely separated in frequency from the response range of the main channel may be used.

In any case, the control arrangement is such that a considerable amount of energy is fed back when the gain is advanced above the maximum level, in order to provide a definite control poten.- tial for the main channel. This is provided in the present example by the preamplifier 32 and. the amplifying stage 31 which precedes. the rectifier circuit connected with the output circuit 4B.

While the invention has been shown and described in its association with a public address system comprising a single microphone and a piurality of loudspeakers, it should be understood. that it may be applied to any similar sound. amplifying system in which acoustic feedback may occur between the sound source and the sound reproducing means.

I claim as my invention:

l. In a public address system, the combination of an electronic tube amplifier comprisinga vari.- able gain amplifier stage having a frequency re.- sponse over a predetermined audio frequency. band including speech frequencies, a second elec.- tronic tube amplifier including signal responsive gain controlling means for said first named amplifier stage and being responsive to a narrow supersonic band of frequencies above normal audibility, an electronic power amplifier coupled jointly to the output. ends of said amplifiers and including a variable overall gain control device for the system, said power amplifier being responsive to both of said frequency bands, a microphone inputV circuit connected with each of said amplifiers, and variable gain control means in said second amplifier' for varying the signal now therethrough with respect to the overall gain of said system.

2. In a public address system, the combination defined in claim l, wherein the first amplifier is provided with a low pass filter network for determining. the upper limit of the frequency re sponse thereof and the second amplifier is provided with a high pass filter network for limiting the low frequency response thereof in said supersonic frequency band.

3. In a public address system, the combinationV defined in claim l, wherein the second amplifier is provided with signal amplitude indicating means for monitoring the overall gain of the system at a level below that at which acoustic feedback may occur.

4. In a public address system, the combination` of an electronic tube amplifier comprising avariable gain amplifier stage responsive to a D.C. controlling potential for varying the gain thereof and having a frequency response over a predetermined audio frequency band including speech frequencies, a second electronic tube amplifier including` rectifier means for deriving from signal flow therethrough a D.C. gain controlling potential for said first named amplifier stage proportional to the signal strength and being responsive to a relatively narrow supersonic band of frequencies above audibility and substantially separated from the first named audio frequency band, a common electronic power amplifier coupled to the output end of each of said amplifiers and including a variable overall gain control device for the system, said last named amplier being responsive to signals in both said audio and supersonic frequency bands, a microphone input circuit connected with each of said amplifiers, and variable gain control means in said second amplifier for varying the signal flow therethrough with respect to the overall gain of said system.

5. An electronic sound amplifying system comprising in combination, a pair of microphones, an electronic amplifier coupled to each of said microphones providing parallel amplifying channels having a common output circuit, a master gain control device in said output circuit, a sound producing device coupled to said output circuit, means in one of said channels for limiting the frequency response thereof within an audible range, means in the other of said amplifiers for limiting the response thereof to a relatively narrow superaudible frequency range in spaced frequency relation to said first named range, automatic volume control means for varying the gain of said first amplifying channel in response to variations in the superaudible signal flow through said second channel, and means for independently adjusting the gain of said second amplifying channel, whereby acoustic feedback is limited to said second channel.

6. An electronic sound amplifying system comprising in combination, a microphone, a loudspeaker and audio frequency power amplifier therefor, an electronic tube amplifier responsive to a predetermined audio frequency band connecting said microphone and power amplifier, a variable gain stage in said amplifier, a variable gain control device at the output end of said amplifier, a second electronic tube amplifier responsive to a higher superaudible frequency band, said last named amplifier having a variable gain control circuit and an output circuit coupled to said first named variable gain control device, a second microphone connected to said last named amplifier stage through said variable gain control circuit, and means providing a gain controlling connection from said output circuit to the variable gain stage in said first named amplifier responsive to increased signal amplitude in the second amplifier for correspondingly reducing the gain in the first amplifier, thereby to limit acoustic feedback through said first named amplifier.

7. An electronic sound amplifying system comprising in combination, a microphone sound pickup device, a loudspeaker sound reproducing device and audio frequency power amplifier therefor having an extended frequency response in a superaudible frequency range, an electronic tube amplifying channel responsive to a predetermined audio frequency band connecting said microphone and power amplifier, a variable gain amplifier stage in said channel, a master gain control device at the output end of said channel, a second electronic tube amplifying channel responsive to a higher and relatively widely separated superaudible frequency band, said last named channel including an electronic tube ampliiier stage having a variable gain input circuit and an output circuit coupled to said first named master gain control device, a second microphone for superaudible energy feedback -pickup from said loudspeaker device connected to said last named amplifier stage through said variable gain input circuit, a signal rectifier connected With said output circuit and having a gain controlling connection with the variable gain amplifier in said first named channel, and means for tuning said output. circuit to said supersonic frequency range, whereby said system may be operated without audible acoustic feedback.

8. In a public address system comprising an electronic tube amplifier having a microphone input circuit and a microphone output circuit, of means for limiting acoustic feedback therethrough comprising a Variable gain amplifier stage in said amplifier responsive to a variable biasing potential, a second electronic tube amplifier having a microphone input circuit provided with variable gain control means and a Y common output circuit with said first named amplifier, a master gain control device in said common output circuit, a lter network in said second amplifier for limiting the response thereof to a predetermined superaudible frequency, and a rectifier device coupled to said second amplifier circuit for deriving a gain controlling potential for said first named variable gain amplifier stage, whereby the gain of said stage is reduced in proportion to the amplitude of supersonic signal energy in said second amplifier above a predetermined threshold value.

9. In a public address system, the combination of a dual channel electronic tube signal amplifier, a common output amplifying means therefor having a common master gain control means therein, a pair of separate microphone input circuits one for each of the signal channels of said dual channel amplifier, means in one of the signal channels of said dual channel amplifier for limiting the response thereof to a predetermined audio frequency range for the transmission of signals through the system, means in the other of said channels of the dual channel amplifier for limiting the response thereof to a narrow supersonic frequency range above said first named range in frequency and substantially widely separated therefrom to convey signals above audibility, gain control means in the second channel of said dual channel amplifier for adjusting the acoustical feedback characteristic of said channel to exceed that in the other of said channels, and means for deriving a gain controlling potential from the second channel of said dual channel amplifier for controlling the gain in the first channel to maintain the gain in the first channel below a maximum gain level without acoustic feedback therethrough, whereby said system is prevented from setting up audible acoustic feedback between the output circuit thereof and the microphone input circuit in response to variations in gain through said system.

JOHN F. STERN'ER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

